Transcript Phylum Cnidaria - cloudfront.net

General Characteristics
• They are radially symmetrical; oral
end terminates in a mouth surrounded
by tentacles.
• They have 2 tissue layers
• Outer layer of cells - the
epidermis
• Inner gastrodermis, which lines
the gut cavity or gastrovascular
cavity (gastrodermis secretes
digestive juices into the
gastrovascular cavity)
• In between these tissue layers is a
noncellular jelly-like material called
mesoglea
Cnidarian Body Plans
Polyp form
• Tubular body, with the mouth directed upward.
• Around the mouth are a whorl of feeding tentacles.
• Only have a small amount of mesoglea
• Sessile
Medusa form
• Bell-shaped or umbrella shaped body, with the mouth is
directed downward.
• Small tentacles,
directed downward.
• Possess a large
amount of mesoglea
• Motile, move by weak
contractions of body
Movement
• The cnidarian body is capable of some kind of coordinated
movement
• Both the epidermis and the gastrodermis possess nerve
cells arranged in a loose network - nerve net, which
innervate primitively developed muscle fibers that extend
from the epidermal and gastrodermal cells
• Stimulus in one part will spread across the whole body via
the network
Nutrition
• Cnidarians are carnivores with hydras and corals
consuming plankton and some of the sea anenomes
consuming small fishes
• They use they tentacles to capture prey and direct it
toward the mouth so that it can be digested in the
gastrovascular cavity via secretions from gland cells
(extracellular digestion); some food is phagocytized by
special cells and digestion occurs intracellularly
• The gastrovascular cavity exists as 1 opening for food
intake and the elimination of waste
• There is no system of internal transport, gas exchange or
excretion; all these processes take place via diffusion.
Stinging Cells
•Prey capture is enhanced by use of
specialized stinging cells called
nematocysts which are individual
cells, usually on the outer surface of
the organism which have a variety of
functions, most usually function in
defense or capture of prey species.
• These stinging cells are sometimes
used to inject toxins which in some
cases are toxic to man, or to entangle
and capture prey.
• When triggered to release, either by
touch or chemosensation, the
nematocyst is released as fast as 700
nanoseconds. After penetration, the
toxin is injected immediately into the
prey paralyzing it.
Reproduction
• One of the most amazing
adaptations is the ability of some
cnidarians to regenerate lost parts or
even a complete body
• Asexual reproduction is common
with new individuals being produced
by budding
• Sea anenomes engage in a form of
asexual reproduction called pedal
laceration
•Fertilization is external, with the
zygote becoming a elongated,
ciliated, radially symmetrical larva planula larva
Planula larva
Cnidarian Taxonomy
Class Hydrozoa
• Includes the solitary freshwater hydra; most are colonial and marine
• Typical life cycle includes both asexual polyps and sexual medusa
stages; however, freshwater hydras and some marine hydroids do not
have a medusa stage
Solitary Hydras
• Freshwater hydras are found in ponds and
streams occurring on the underside of
vegetation
• Most possess a pedal disc, mouth,
hypostome surrounded by 6-10 tenetacles
• Mouth opens to the gastrovascular cavity
• The life cycle is simple: eggs and sperm
are shed into the water and form fertilized
eggs; planula is by passed with eggs
hatching into young hydras
• Asexual reproduction via budding
Class Hydrozoa cont.
Colonial Hydrozoans - e.g., Obelia
• Possess a skeleton of chiton that is
secreted by the epidermis
• All polyps in the colony are usually
interconnected
• Two different kinds of individuals
that comprise the colony: feeding
polyps or gastrozooids (C) and
reproductive polyps or gonozooids
(B)
Class Hydrozoa cont.
Life Cycle of Obelia
• Gonozooids release free swimming medusae
• Zygotes become planula larvae, which eventually
settle to become polyp colonies
• The medusae of
hydroids are smaller
than those of
jellyfishes (C.
Scyphozoa)
• Also, the margin of
the bell projects
inward forming a
shelf-like velum
Class Hydrozoa cont.
Other Hydrozoans
Portuguese man-of-war:
Single gas-filled float
with tentacles
Tentacles house the
polyps and modified
medusae of the colony
Class Scyphozoa
Jellyfish
• The medusae are large and
contain massive amounts of
mesoglea
• The differ from the
hydrozoan medusa in that the
lack a velum
•Possess four gastric pouches
lined with nematocysts; these
are connected with the mouth
an the gastrovascular system
Scyphozoan Life Cycle - Aurelia
• Gametes develop in
gastrodermis of gastric pouches;
eggs and sperm are shed
through mouth
• Fertilized eggs develop into a
planula larva; settles on
substrate and develops into a
polyp - scyphistoma
• Scyphistoma produces a series
of polyps by budding - strobila
• The polyps undergo
differentiation and are released
from the strobila as free
swimming ephyra
• Ephyra matures into an adult
jellyfish
Class Anthozoa
• Exclusively marine; there is no medusa stage
• At one or both ends of the mouth is a ciliated groove called the
siphonoglyph; generates a water current and brings food to the
gastrovascular cavity
• Possess a well developed pharynx
• The gastrovascular
cavity is large and
petitioned by septa or
mesenteries; increase
surface area for digestion
or support
• Edges of the septa
usually have threadlike
acontia threads,
equipped with
nematocysts and gland
cells
Class Anthozoa cont.
• Solitary anthozoans
include sea anemones
• Most anthozoans are
colonial (e.g. corals) and
secrete external skeletons
composed of calcium
carbonate.
• Corals obtain much of
their energy from
microscopic photosynthetic
green algae (zooxanthellae)
or dinoflagellates that live
symbiotically inside the
cells of the coral